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Hefner made the announcement through Twitter. "When I gave Crystal the ring, she burst into tears,” he wrote. “This is the happiest Christmas weekend in memory."

He later wrote on Twitter: "Yes, the ring I gave Crystal is an engagement ring. I didn't mean to make a mystery out of it. A very merry Christmas to all."

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This will mark Hefner’s third marriage. He was first married to Mildred Williams in 1949 until it ended 10 years later. Hefner later wed Playmate of the Year Kimberly Conrad in 1989. The couple separated in the late ’90s and their divorce came earlier this year, according to Playboy Enterprises Web site.

Harris was Miss December for Playboy in 2009, according to Reuters.

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Hefner once told People magazine last year that marriage for him wasn’t out of the question. "It's unlikely since I haven't been very successful at it,” he said. "But anything is possible."